Modern hard corals (Class Hexacorallia; Order Scleractinia) are widely studied because of their fundamental role in reef building and their superb fossil record extending back to the Triassic. Nevertheless, interpretations of their evolutionary relationships have been in flux for over a decade. Recent analyses undermine the legitimacy of traditional suborders, families and genera, and suggest that a non-skeletal sister clade (Order Corallimorpharia) might be imbedded within the stony corals. However, these studies either sampled a relatively limited array of taxa or assembled trees from heterogeneous data sets. Here we provide a more comprehensive analysis of Scleractinia (127 species, 75 genera, 17 families) and various outgroups, based on two mitochondrial genes (cytochrome oxidase I, cytochrome b), with analyses of nuclear genes (ß-tubulin, ribosomal DNA) of a subset of taxa to test unexpected relationships. Eleven of 16 families were found to be polyphyletic. Strikingly, over one third of all families as conventionally defined contain representatives from the highly divergent “robust” and “complex” clades. However, the recent suggestion that corallimorpharians are true corals that have lost their skeletons was not upheld. Relationships were supported not only by mitochondrial and nuclear genes, but also often by morphological characters which had been ignored or never noted previously. The concordance of molecular characters and more carefully examined morphological characters suggests a future of greater taxonomic stability, as well as the potential to trace the evolutionary history of this ecologically important group using fossils.

sequenced as described in ref. 18. Internal primers McytbseqF (5 0-ATT GAC TAT GGC GAC CGC TTT T-3 0) and McytbseqR (5 0-GAA TAA AAT TCT CTG CGT CTC C-3 0) for cytB were used in addition to PCR primers. Primers to amplify the b-tubulin gene (intron and exon regions), designed on the basis of sequence data of Montastraea faveolata 19 , were TubulinF (5 0-GCA TGG GAA CGC TCC TTA TTT-3 0) and TubulinR (5 0-ACA TCT GTT GAG TGA GTT CTG-3 0). They amplify a region corresponding to amino acid positions 144-299 within exon 4 of the human and Drosophila b-tubulin gene; the beginning of the intron corresponds to position 247, and the flanking exons have 99% amino acid similarity to the corresponding vertebrate sequence. Depending on the genus, one, two or three bands of about 600 bases, 1.0-1.5 kilobases (kb) and more than 2.0 kb were amplified by PCR with the above-described protocol. The size difference between bands was due to the length of the intron. Because most genera had a 1.0-1.5-kb band, this was used for phylogenetic analyses. Diploastrea and Solenastrea had only the 600-base-pair (bp) band, and no bands could be amplified for Acanthastrea rotundoflora and Favites chinensis; consequently these four taxa were not analysed for b-tubulin. Amplified fragments of the b-tubulin gene were separated by agarose electrophoresis, cloned with the pGEM-T System (Promega) and sequenced for both strands. At least five clones obtained from each of two independent PCRs were analysed. If only one sequence occurred more than once, this sequence was used in the phylogenetic analyses; otherwise the two most abundant sequences were used. DNA phylogenetic analyses Only the exon regions of the b-tubulin gene (444 bp) were analysed, because the intron was too variable for alignment. Phylogenetic analyses were performed with PAUP* 20. DNA sequences of the entire cytB gene and the COI gene excluding the third codon position (total length 1,557 bases) were combined on the basis of nucleotide saturation analyses and the incongruence length difference test. Phylogenetic trees were constructed on the basis of neighbour-joining (NJ), maximum-parsimony (MP) and maximum-likelihood (ML) methods with the use of PAUP*. The NJ analysis was done with a two-parameter model 21. In MP and ML analyses, heuristic searches with TBR branch swapping and 25 random additions of taxa were performed. For ML analysis, we used Modeltest 22 to find an appropriate model of evolution. For mitochondrial genes the K81uf model 23 with gamma parameter (G) and proportion of invariable positions (I) was chosen. For b-tubulin we chose the TrN model 24 with G and I. Bootstrapping was used to evaluate support for trees (1,000 replicates for NJ and MP; 300 bootstraps with the fast-stepwise heuristic search for ML).

The first scleractinians, progenitors of modern corals, began to appear 240 million years ago; by the late Jurassic (150 Ma) most families of modern corals had evolved and begun forming reefs (1, 2). Mechanisms controlling the recruitment of new corals to sustain these structures are, however, poorly understood (3). Corals, like many marine invertebrates, begin life as soft-bodied larvae that are dispersed in the plankton (3, 4). As the first step in developing a calcified coral colony, the larva must settle out of the plankton onto a suitable substratum and metamorphose to the single calcified polyp stage cemented to the reef (3, 5). Our analyses of the metamorphic requirements of larvae in divergent coral families surprised us by revealing the existence of a common chemosensory mechanism that is required to bring larvae out of the plankton and onto the reef. This mechanism appears to be quite old, predating both the phylogenetic divergence of these coral families and the development of different modes of coral reproduction.

Stored lipids in marine planktonic larvae play an important role in buoyancy and as an energy source and thus are a key to understanding the dispersal and settlement potential of larvae. However, little is known about lipid content and composition in different coral species or their temporal changes during larval dispersal. We examined the lipid content and composition of eggs and planula larvae of Acropora tenuis, a reef-building coral, and their temporal changes over the course of larval dispersal and settlement. The total content and composition of lipids in newly released planulae of the brooding corals A. brueggemanni, Pocillopora damicornis and Heliopora coerulea were also examined for comparison. A. tenuis eggs were positively buoyant; lipids accounted for 85% of their dry weight, but decreased to 50% of their dry weight within 30 d after spawning. Wax esters were a major component of lipids in the eggs; they decreased appreciably by 5 d after spawning and decreased thereafter. In contrast, the phospholipid content remained almost constant. The period of rapid decrease in wax esters occurred before settlement, suggesting that A. tenuis planulae consume mainly wax ester lipids as an energy source during the planktonic phase. In contrast, the lipid content of H. coerulea planulae, which have a shorter dispersal period, was lower (41% of dry weight) than that found in 5-d-old planulae of A. tenuis and newly released planulae of A. brueggemanni and P. damicornis (> 58% of dry weight). Triacylglycerols in lipids were detected in P. damicornis and H. coerulea planulae, which settle quickly after release, but not in A. tenuis or A. brueggemanni, suggesting that triacylglycerols are used for rapid energy. These findings suggest that lipids are used for buoyancy and as an energy source and are related to differences in the dispersal period of planulae among coral species.

No abstract

No abstract

No abstract

Natural hybridization of corals in the Indo-Pacific has been considered rather rare. However, field studies have observed many corals with intermediate interspecific or unusual morphologies. Given that the existence of F1 hybrids with intermediate interspecific morphologies has been proven in the Caribbean, hybrids may also inhabit the Indo-Pacific and occur more frequently than expected. In this study, we focused on two morphologically different species, Acropora florida and A. intermedia, and performed crossing experiments at Akajima Island, Japan. Results showed that these species could hybridize in both directions via eggs and sperm, but that fertilization rates significantly differed according to which species provided eggs. These results are similar to those reported from the Caribbean. Although all embryos developed normally to the planular larval stage, the developmental processes of some hybrid embryos were delayed by approximately 1 h compared with conspecific embryos, suggesting that fertilization occurred 1 h later in interspecific crosses than in intraspecific crosses. More successful hybridization could occur under conditions with low numbers of conspecific colonies. Additionally, a comparison of survival rates between hybrid and intraspecific larvae revealed that intra- and interspecific larvae produced from eggs of A. florida survived for significantly longer than those produced from eggs of A. intermedia. Considering these data, under specific conditions, hybrids can be expected to be produced and survive in nature in the Pacific. Furthermore, we identified one colony with intermediate morphology between A. florida and A. intermedia in the field. This colony was fertilized only by eggs of A. florida, with high fertilization rates, suggesting that this colony would be a hybrid of these two species and might be backcrossed.